Electrical wave filter



March 24, 1936. n. F. CICICOLELLA ELECTRICAL WAVE FILTER Filed Aug. 26, 1932 waving-$507 lvmssm'slvveu lNlfEA/TOR v 0. COLELL'A Arromvzk Patented Mar. 24, 1936 UNITED STATES nmc'rmoar. WAVE mrlllt David F. Ciccolella, Albany, 'N.'Y., alslgnor to Bell Telephone Laboratories,

Incorporated,

New York,-N. Y., a corporation of New York Application August 26, 1932, No. 630,528 lrcloimo, (01. 179-78) This invention relates to wave transmission and more particularly to electric wave filters adapted for use in a loop circuit which also carries longitudinal currents.

The principal object is to pass without appreciable distortion a range of frequencies in the loop circuit and at the same time to suppress the longitudinal currents over a range of frequencies.

' A feature of the invention is amulti-section d filter in which each shunt branch is providediw th a separate ground connection.

- An aerial telephone line which parallels an aerial power line is exposed to electrostatic and .electromagnetic induction ,which tends to set up 15' longitudinal currents in the telephone line.

this way disturbances in the power line produced by switching, changes of load, accidental grounds, lightning or the. like cause corresponding disturbances in the telephone line. These surges on the 20 telephone line may, in turn, be, reradiated to other communication systems, such as .a nearby radio antenna, causing noise or other objectionable interference. Under these circumstances there is presented theproblem of suppressing 25 the undesirable longitudinal currents without appreciably interfering with the transmission of. the signal currents in the loop circuit.

In one instance in which this problem arose suppression of the longitudinal currents in the telephone line was attempted by inserting a single-section, low pass filter having a single shunt branch in the telephone line'at apoint some distance from the affected antenna. The -filter was of the balanced-to-ground' type, with 35 the electrical mid-point ofthe shunt impedance connected to a metal rod driven into the ground. r The cut-off point of the filter was so chosen that carrier telephone currents below 30 kllocycles were transmitted but the longitudinal currents above 40 kilocycles were attenuated and their ,terference in the antenna circuit was thus ei I duced. But tests showed that the longitudinal the vicinity of the inter oilcred an appreciably 12 may be largelyovercome;

high impedance. The straps connecting the midpointsof the shunt arms of the three-section filter were then removed and a separate ground connection to an individual metal rod driven into the ground was provided for each' shunt branch. 5 When this had been done the niulti-section' filter 'gave a satisfactory performance and the longitudinal currents were sufficiently suppressed.

The nature of the invention will bemore fully understood by reference to the following detailed description, taken in connection with the accompanyingdrawing in which: A I

Fig. 1 shows an embodiment of the invention in a wave transmission system;

Fig. 2 is a detail of the shunt condensers of the filter of Fig 1, illustrating the separate grounding arrangement; I

Figs. 3 and 4 are circuit diagrams to which reference is made in explainingthe invention; and

Fig. 5 shows typical attenuation characteristics obtainable with the structures of Figs. 1, 3- and 4.

In Fig. l a power-line II or other source of disturbance is.shown locatedin such proximity to a portion of a twowire telephone line 12 that electrical disturbances in the power line will induce'longitudinal currents of corresponding fre-' quency in the. telephone circuit.

By carefully balancing the two sides and by transposing the .two wires in a well known manner the eflect of these disturbances on the loop circuit of the line There remains, however, the longitudinal current which traverses both wiresof the line II in the same' direction, with ground as theretum path. A radio receiving antenna "is shown adjacent to a portion of the line l2. Disturbances on the telephone line of the frequency used'in the radio channel tend to be radiated tothe antenna and to caus interference with the received radio signals, either 40 directly or by modulation. To suppress the undesired longitudinal currents in that portion of the telephone circut l2 which is near the antenna 13, anelectric wave filter ll is inserted in thetelephone hne. The filter I4 is .of the lowepass' type, with a balanced-to-ground construction. and comprises two full sections of the type illustrated and described in the patent to Campbell, 1,227,113 of May 22, 1917, to either end of which is added a 'half section of the type disclosed in Patent 1,493,600, also to Campbell, issuedj'mayils, 1924. 'The electrical mid-points of theshunt branches are brought to separate ground terminals l5, l6 and II, which are connected, respectively, to

the individual grounds G3, a the G3. These shows a typical 2 a,ost,ae1

grounds may be metal stakes driven into the earth, buried metal plates, orother suitably confilter ll comprising the three pairs of condensers be spaced apart'a distance which is dependent in the shunt arms and the ground connections.

The ground terminals l5, l8 and I! are connected to separate metal plates G1, G1, and Gs buried in 10 the earth at some distance from each other.

Thereason for having separate grounds 01, G:

and G: for the different shunt branches of the filter will now be explained, with the aidof the diagramsshown in Figs. 3 and 4. When a ground of the type described is used in a dry or rocky terrain it will be found that there is always associated therewith an impedance of appreciable magnitude. In the instance referred to above, the value of this impedance was found by measurement to be as high as 100 ohms in certain places. This ground resistance is shown schematically in Figs. 3 and 4, as R.

In accordance with former practice the three ground terminals l5, l6 and I1 would be strapped together and connected to a single ground. The filter It would then present to the longitudinal circuit the configuration shown schematically in Fig. 4 inwhich R represents the ground resistance, 2. is the impedance facing the input end of the filter, Zr is the impedance into which the filter works at the output end and E represents the electromotive force acting between the two wires of line 12 and ground. Curve A of Fig. 5 shows a typical transmission characteristic obtainable with the network of Fig. 4. The transmission loss in decibels is given by the ordinates and the a frequency in kilocycles per second is represented by the abscissas. It will be noted that the maximum loss is below 50 decibels and at 100 kilo.- cycles the curve falls to the vicinity of 10 decibels.

The reason that the loss falls to such a low value .50 grounds, thus, in effect, reducing the value of the ground impedance R. If, for example, three different ground connections are used in parallel, while terminals l5, l0 andll remain strapped together, the value of R in Fig. 4 is reduced to 65 one-third of its former magnitude, and the trsnsmission characteristic of the filter will be as shown by curve B,of Fig. 5.- Curve-B itwillbe' noted indicates an increase of approximately ten decibels over the corresponding poinis on curve A.

.60 In the arrangement of the invention each shunt arm of the filter is grounded separately, as shown schematicalyinFlg.3,andinthiswaythepath through the ground wire in the filter to the load is effectively open circuited. Curve C of Fig. 5

by the filter of .3, which is identical to the filter of Fig. 4 except for the separate grounding arrangement noted. It is seen that curve C reaches a maximum loss of approximately '15 70 decibels and at no point in the attenuating range on characteristic given of a separate ground for each shunt impedance arm. In order that the conduction between the various ground terminals II, it and I1 will not be too large and thereby defeat the purpose of the separate grounding, it is necessary that the 5 individual ground rods or plates (31, G: and G:

a'wave filter of balanced construction comprising series arms andv shunt arms, said filter being located in said telephone, line and the electrical mid-point of each shunt arm of said'filter being 2 connected to a separate ground, whereby the impedance of the electrical path formed by the ground connections between any ta of said electrical mid-points is made of sufiicient magnitude to prevent the tendency to short-circuit the series 25 arms of said filter. 2. In a signal transmission system, a signal circuit comprising a pair of line wires balanced with respect to ground, a longitudinal circuit comprising said pair of line wires as one path and 30 ground'as a return path, and a balanced wave filter having series branches in each of said line wires and a plurality or branches in shunt with respect to said line wires, the mid-point of each of said shunt branches being connected to a sepa- 35 rate ground, whereby the'impedance of the electricalpath formed by the ground connections between said mid-points is made so high that the effective short-circuiting of the series branches of said filter is prevented. I 40 -3. In an electrical communication system, a

' transmission circuit consisting of a telephone line as one path and'groimd as the return path, and frequency selective means in said circuit comprising a wave filter oi the ladder type having a plurality of shunt impedances in which the electrical mid-point of each shimt impedance-is connected to a ground, whereby th'e'short-' circuiting eifect caused by the ground connections between said mid-points is substantially elimis 4. A two-wire signaltrensmission circuit. in. which longitudinal currents are'fiowing in the samedirectionineachwire'ofsaidcircuitwith groundasthereturnandsuppressionmeansinssidcircuitoonslstingotawavefilteroftheseries shunt type comprising-a plurality of shunt gbrancheatheelectrical mid-point of each of said.

shlmtbranchesbeingconnectedtoanindividual ground, whereby the conductance of the electrical pathbetween two of said mid-points comprls-' ing the ground connections is greatly 5. A signal circuit oomprisinga pairof line .wires, a longitudinal circuit peir ofwiresasonepath-withgroimdastherehim path, andfrequency-selec tivemeans'locatedin said signal circuit-consisting of a wave filter oomprising'seriesimpedsneesandshimtimpedmees, themid-pointoteachofsaidshimtimpedsmes being connected to a whereby theimpeda'nee'of theelectrical path formedby thegroundconnectionsbetweentwoofssideleetrical mid-points is made suificiently large in m altime to avoid the efiec'tive short-circldting 1s" '7; In a wave transmission system, a two-wire signal circuit in which undesired longitudinal currents are induced, and means located in said signal circuit for suppressing said longitudinal currents consisting of a balanced wave filter comprising serles'impedances and shunt impedances, the mid-point of each 01' said shunt impedances being connected to a separate ground, in order to increase the impedance of the electrical path between two of said mid-points comprising the ground connections.

8. In a wave transmission system, a two-wire circuit in which undesired longitudinal currents are induced, a balanced wave filter comprising series impedances and shunt impedances located in said two-wire circuit, the electrical mid-point of each of said shunt impedances being connected to a separate ground, whereby the short-circuiting effect upon a series impedanceof said filter caused by the conductance between two of said mid-points through the ground connections is substantially eliminated.

9. In a wave transmission system comprising a loop circuit in which undesired longitudinal currents are also flowing, with ground as the return path, a wave filter of the series-shunt type comprising series irnpedances and a plurality of shunt impedances located in said loop circuit, said filter-being adapted to attenuate said longitudinal currents and. said filter having the electrical mid-point of each shunt impedance connected to a separate ground, whereby the impedance of the electrical path formed by the ground connections between any two of said electrical mid-points is made 01 suflicient magnitude to prevent the tendency to short-circuit the series impedances of said filter.

10. In a signal transmission system, an openwire telephone line in which longitudinal currents are flowing with groluid as the return path, and frequency selective means located in said telephone line consisting of a balanced wave filter comprising impedances in series with the line and a plurality of impedance branches in shunt with the line, the electrical mid-point of each of said shunt branches being connected directly to a separate metal plate, and each of said plates being buried in the ground at a minimum distance of the order of five feet from said other plates.

11. In an open-wire signal transmission system, a signalcircuit comprising a pair of line wires balanced with respect to ground, a longitudinal circuit comprising said pair of line wires as one path and ground as the return path, a source of disturbing voltage tending to cause undesired currents to flow in said longitudinal circuit, and a balanced wave filter having series branches in each of said line wires and a plurality of branches in shunt with respect to said line wires, said filter a being adapted to attenuate saidundesired currents, the electrical mid-point .of each of said slumt branches being connected directly to a separate ground plate, and each of.s aid plates being buried in the earth at spaced intervals from each other. T

DAVID F. CICCOLELLA. 

